Aluminum alloy



United States Patent This invention relates in general to aluminum alloys and in particular to aluminum alloys of high fracture toughness and high crack growth resistance under cyclic loading conditions.

Presently an aluminum alloy used in airframe construction with the trade designation of type 7178 has the following compositional range: zinc, 6.3 to 7.3%; mag nesium, 2.4 to 3.1%; copper, 1.6 to 2.4%; iron, 0.75% maximum; silicon, 0.50% maximum; manganese, 0.30% maximum; chromium, 0.18 to 0.40%; titanium, 0.20% maximum; all other extraneous elements, 0.05% maximum each; total limit for all other extraneous elements, 0.15% maximum; with the balance being aluminum. 7178 aluminum is a strong, high strength material having moderate temperature applications. This alloy was intended primarily for advanced subsonic aircraft applications.

When being used on subsonic aircraft, 7178-T6 aluminum has exhibited a definite shortcoming due to a very brittle behavior when small cracks and flaws were present in the material. Broken parts, large cracks and separations have been observed on airframe structures comprised of 7178-T6 aluminum due basically to this brittle behavior. The application of cyclic stresses to this alloy results in the nucleation and rapid propagation of fatigue cracks from various small faults in the material. As the crack size increases, the amount of load on the 7178 aluminum specimen needed for failure becomes significantly less than the remaining uncracked net section would indicate. The terminology applied to this performance of the 7178 aluminum is known as a lack of fracture toughness and crack growth resistance.

Obviously this performance is a serious drawback to a material when contemplated for use on current advanced subsonic flight applications in airframe structures. Therefore, we concluded that the fracture toughness and crack growth resistance of 7178 aluminum should be greatly improved before further use in aircraft structures.

It was our conviction that any program achieving a 7178 aluminum alloy with improved properties of fracture toughness and crack growth resistance must be based either upon the metallurgical operation of the alloying and residual elements in the aluminum matrix, upon production and fabrication procedures or upon the combination of both of these approaches. Salient literature as well as related experiments suggested to us that compositional variations would be the most fruitful approach for control of fracture properties in this alloy system. This approach required the supposition that some impurities and alloying additives were beneficial in their influence on fracture properties as opposed toother impurities and alloying additives which were harmful. Our object was to eliminate or reduce the harmful ranges of alloying elements and the detrimental influence of impurities on fracture toughness and crack growth resistance without an undue sacrifice of other existing attractive properties.

After the metallurgical influence of each alloying addition had been ascertained, these ideas were evaluated in light of actual performance of laboratory produced test alloys. Following testing of laboratory produced material for fracture and mechanical properties, prefer-red alloy compositions were formulated so that the crack growth resistance and fracture toughness were optimized without sacrificing other desired properties.

Our findings indicate that several alloying and residual elements found in 7178 aluminum affect fracture toughness. As a result of our research, a very approximate formula which may be used to measure longitudinal grain fracture toughness in the 7178 alloy compositional range is as follows: (for 0.16 gage only) Other common elements found in the 7178 aluminum matrix appear to be more subtle in their effect on fracture toughness, with the possible exception of silicon.

The data compiled during our research with regard to crack growth rate would indicate that increases in copper content would decrease the crack growth rate under cyclic loading while increases in the Zinc content would increase the growth rate. Iron appeared to have a variable effect; increased iron slowing the crack growth rate for short cracks and low cyclic stresses and accelerating the crack growth rate for long cracks and high cyclic stresses.

Our research has made possible the design of two new alloys which have properties related to 7178 aluminum except for superior performance in relation to the properties' of crack growth resistance and fracture toughness. In comparison with standard 7178 aluminum, our alloy A would exhibit excellent fracture toughness but comparable crack growth rates while alloy B would exhibit more desirable crack growth rate properties and good fracture toughness.

Alloy A Alloying element: Percentage (wt.) Zinc 6.3 to- 6.7 Magnesium 2.4 to 2.7 Copper 1.6 to 2.0 Silicon, maximum 0.1 Iron, maximum 0.1 Manganese, maximum 0 l Chromium 0.18 to 0.2%

Aluminum, balance.

Alloy A and Alloy B differ in composition from 7178 aluminum in the following critical elements: zinc, magnesium, copper, iron, silicon, manganese, chromium and titanium. The compositional changes in the above alloys are of three types: narrowing the existing range for zinc and magnesium and chromium, splitting the existing range for copper, and lowering the maximum limit for iron, silicon, manganese and titanium. The following discussion will present the influence of these alloying elements and residual elements on the fracture properties in 0.16 gage alloys.

Increasing the zinc as an alloying addition in a 7178 aluminum matrix results in a decrease of the fracture toughness, an increase in the crack propagation rate and an increase in .the tensile properties. Zinc combines with magnesium to form the hardening and strengthening precipitate of MgZn As zinc is basic to the desirable tensile properties of a 7178 aluminum matrix, a forced compromise is necessary between the fracture property and the uncracked strength (tensile) property. Therefore the zinc range of 6.3 to 6.7 weight percent was selected for Alloys A and B so that the tensile properties would be adequate to meet current aircraft specifications, but the alloy would still have optimum fracture characteristics.

There are two primary functions performed by magnesium in a 7178 aluminum matrix. The first function is the forming of hardening precipitates of the MgZn type and the second function is acting as a solid solution strengthener. Some magnesium also combines with silicon in a 7178 aluminum matrix thus forming the unwanted intermetallic constituent Mg Si. As the magnesium content is increased in a 7178 aluminum matrix, the fracture toughness decreases, thus once again a compromise must be made between the tensile properties and the fracture properties. The 2.4 to 2.7% magnesium content called for in Alloys A and B has been found to be consistent with the required strength properties and will maximize fracture toughness of the aluminum matrix.

Copper as an alloying addition to 7178 type aluminum has the general effect of slightly increasing the yield strength and the tensile strength but substantially diminishing the fracture toughness. A high iron content tends to emphasize the reduced fracture toughness resulting from a high copper concentration due to the formation of the intermetallic Al Cu Fe. Most copper in the 7000 series aluminum alloys goes into the formation of intermetallic compounds (primarily AI Cu Fe) and hardening precipitates, with the remainder going into solid solutions. The copper content for the standard 7178 aluminum is 1.6 to 2.4%. After considerable tests, we decided to split the previous range into two different ranges of copper concentration: 1.6 to 2.0% for Alloy A and 2.0 to 2.4% for Alloy B. The lower copper concentration results in an alloy with the highest possible fracture toughness but slightly reduced strength and crack growth properties. This alloy allows maintenance of most yield and tensile strength properties in comparison with the 7178 aluminum plus having the improve fracture toughness. An example of the reduction is the fact that the transverse yield strength of Alloy A would be about 1000 p.s.i. less than 7178 aluminum. Alloy A would be used in situations where a crack length of maximum size is desired before the final failure of the material will occur. Alloy B has the higher copper concentration which sacrifices some fracture toughness for crack growth resistance and general strength properties. We feel the separation of the allowed copper composition in 7178 aluminum into two different alloying ranges represents an advantage for specialized usage of the aluminum on high speed aircraft.

Iron as a residual element in 7178-type aluminum causes the formation of intermetallic compounds, principally AI Cu Fe and a AlFeSi, but iron has little effect on mechanical properties. The intermetallics formed by the presence of iron can apparently either arrest or accelerate fatigue crack propagation depending upon the size of the intcrmetallic particle, the stress level imposed, and the length of fatigue crack present, but the intermetallics are found to drastically reduce the strength of the alloy when cracks are present. Because of the reduction in strength, the iron content in Alloys A and B was restricted to below a weight percentage of 0.1%.

Silicon as a residual element in a 7178 aluminum must be considered in conjunction with iron and magnesium because of the intermetallics 0c AlFeSi and Mg Si which form when silicon is present. Our research indicates that increased silicon, despite the increase in volume fraction of intermetallic constituents which occurred, was beneficial toward fracture toughness. Unfortunately increased concentrations of silicon markedly decrease tensile properties. Therefore, this element should be restricted to 0.1 weight percent at the maximum.

In addition to the above conclusions derived from our research, we have made the following general observations with regard to the fracture properties of 7000 series aluminum alloys. The coarser the grain size of the alloy is, usually the faster the crack propagation rate of the alloy is. Large intermetallic constituents normally seem to have an adverse effect on fracture toughness. Intermetallic constituents appear to have an effect on crack growth rate under cyclic loading. The magnitude and type of effect appears to depend upon the particle size, concentration, stress level and fatigue crack length. In line with the latter observations we have reduced the allowed maximum content of manganese and chromium in Alloys A and B because of the possibility of forming large intermetallic particles if these two alloys are present in larger quantities.

The expression G is often used as a measure of fracture toughness.

GCZ'IT/ E 0' aol, where: ag the gross area stress in a rectangular center notched panel, a=one half the crack length, E=Youngs Modulus; 10.3 10 for aluminum, a=correction factor for specimen configuration. The factor is very nearly 1.0 for specimens commonly used.

As can be seen G is directly proportional to the length of crack the material will tolerate before its catastrophic extension (failure) under a constant stress situation. A typical longitudinal grain G value for 7178-T6 aluminum is 130. An average value of G for Alloy A is 260 which represents a improvement in fracture toughness. Perhaps even more significant is the increase in minimum values. G values for some specimens of 1778-T6 can drop as low as 50 whereas the minimum value for Alloy A is 180, an increase of over 300 percent. G values for Alloy B are about 35 points lower than for Alloy A.

TABLE 1 Go along the Longitudinal Grain 7178-T6 Alloy A Alloy B All fracture properties described in the above table and discussion were made using the standard T6 heat treatment for aluminum alloy 7178 and apply to 0.16 gage only.

We claim:

1. An aluminum alloy of high fracture toughness and crack growth resistance consisting essentially of by weight percent as follows: zinc, 6.3 to 6.7%; magnesium, 2.4 to 2.7%; copper, 1.6 to 2.0% iron, 0.1% maximum; silicon, 0.1% maximum; manganese, 0.1% maximum; chromium, 0.18 to 0.28%; titanium, 0.1% maximum; all other extraneous elements, 0.05% maximum for each; total limit for all other extraneous elements, 0.15% maximum; and the balance aluminum.

5 6 2. An aluminum alloy of high fracture toughness and References Cited by the Examiner crack growth resistance consisting essentially of by weight UNITED STATES PATENTS percent as follows: zinc, 6.3 to 6.7%; magnesium, 2 4 to 2,240,940 5/1941 Neck 2.7%; copper, 2.0 to 2.4%; IIOH, 0.1% maximum; silicon, 2,301,759 11/1942 Stroup 0.1% maximum; manganese, 0.1% maximum; chromium, 5 2 7 31 5/1945 Gauthier 75 141 0.1.8 to 0.28%; titanium, 0.1% maximum; all other eX- 2,403,037 7/1946 Zeigler et al. 75l4l traneous elements, 0.05% maximum each; total limit for all other extraneous elements, 0.15% maximum; and the DAVID RECK Prmary Exammen balance aluminum. 10 R. O. DEAN, Assistant Examiner. 

1. AN ALUMINUM ALLOY OF HIGH FRACTURE TOUGHNESS AND CRACK GROWTH RESISTANCE CONSISTING ESSENTIALLY OF BY WEIGHT PERCENT AS FOLLOWS: ZINCE, 6.3 TO 6.7%; MAGNESIUM, 2.4 TO 2.7%; COPPER, 1.6 TO 2.0%; IRON, 0.1% MAXIMUM; SILICON, 0.1% MAXIMUM; MANGANESE, 0.1% MAXIMUM; CHROMIUM, 0.18 TO 0.28%; TITANIUM, 0.1% MAXIMUM; ALL OTHER EXTRANEOUS ELEMENTS, 0.05% MAXIMUM FOR EACH; TOTAL LIMIT FOR ALL OTHER EXTRANEOUS ELEMENTS, 0.15% MAXIMUM; AND THE BALANCE ALUMINUM. 